In wavelength division multiplexed (WDM) optical communication systems, a number of different optical carrier wavelengths are separately modulated with data to produce modulated optical signals. The modulated optical signals are combined into an aggregate signal and transmitted over an optical transmission path to a receiver. The receiver detects and demodulates the data.
One type of modulation that may be used in optical communication systems is phase shift keying (PSK). According to different variations of PSK, data is transmitted by modulating the phase of an optical wavelength such that the phase or phase transition of the optical wavelength forms a symbol representing one or more bits. In a binary phase-shift keying (BPSK) modulation scheme, for example, two phases may be used to represent one bit per symbol. In a quadrature phase-shift keying (QPSK) modulation scheme, four phases may be used to represent two bits per symbol. Other PSK formats include amplitude phase shift keying (APSK) and differential phase shift keying (DPSK) formats and variations of PSK and DPSK formats, such as return-to-zero DPSK (RZ-DPSK) and polarization division multiplexed QPSK (PDM-QPSK).
Quadrature amplitude modulation (QAM) broadly describes a modulation format wherein data is represented using phase shift keying with or without amplitude shift keying. For example, a 16-QAM modulation format uses phase shift keying and amplitude shift keying to represent four bits per symbol. PSK modulation schemes may be broadly viewed as QAM schemes and may be referred to as a level of QAM. For example, BPSK may be referred to as 2QAM and QPSK may be referred to as 4QAM.
Data bits are mapped to QAM signals according to a signal constellation. A signal constellation is a predetermined plan or map indicating how information bits correspond to associated symbols modulated on an optical signal. The constellation is typically represented as a two-dimensional scatter diagram in the complex plan. The real and imaginary axes of the complex plane often called the in-phase, or I-axis, and the quadrature, or Q-axis, respectively. For a particular modulation format, the constellation identifies the exact information bits that correspond to each symbol (having real and imaginary values) modulated on an optical wavelength.
One problem associated with optical communication systems is maintaining the integrity of the data being communicated, particularly when optical signals are transmitted over long distances in long-haul communication systems. Accumulated noise contributed by many different sources in a transmission path may cause degradation of the signals and may cause difficulty in differentiating between the binary digits (i.e., the ones and zeros) in a data stream.
Forward Error Correction (FEC) is a technique used to help compensate for this degradation. FEC is essentially the incorporation of a suitable code into a data stream at the transmitter. The transmitter receives a data stream and encodes the data stream using an FEC encoder that introduces some redundancy in the binary information sequence of the data stream. The receiver receives the encoded data and runs it through an FEC decoder to detect and correct errors.
When an FEC code is combined with a modulation format, the system may be described as including a coded modulation. One example of a coded modulation is known as bit-interleaved coded-modulation (BICM). In a BICM scheme FEC coding is applied to a data stream and the FEC coded data stream is then bit-interleaved (i.e. the order of the bits is permuted). The coded and interleaved data stream is then modulated according to a selected data modulation. The performance of BICM can be further increased in some cases by exchanging information between the de-mapper and the decoder and performing iterative decoding (ID). BICM schemes with ID decoding are known as BICM-ID schemes. A modified BICM-ID coded modulation scheme is described in U.S. Pat. No. 8,775,892 (the '892 patent), the teachings of which are hereby incorporated herein by reference.